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Old 3rd May 2008, 08:47 AM   #1
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Default Leach Super Amp Power Supply

Hi,
The Leach Super Amp is my first diy audio project that I'm doing for my Junior Design class. Being in college and therefore broke, after 230 dollars into the project I'm left without cash, and this point time, to buy a transformer. I've read that 30 volts drives the amp at high enough frequency. I have a +/- 40 volt transformer. Would this be sufficient enough to demonstrate the operation of the amplifier during my final presentation? Ultimately I would buy an 85 volt transformer I just need something to be able to display the amps operation once during class. Also, with this voltage, resistors 13 and 14 go to zero. Should I install a jumper or a small ohm resistor to obtain a current? My other option is to use a variac to obtain the required voltage and forgo the decoupling. Any comments or ideas? Thanks.
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Old 3rd May 2008, 09:16 AM   #2
AndrewT is offline AndrewT  Scotland
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firstly, learn the difference between AC voltage and DC voltage or your grades are going to look pretty poor.
Secondly, learn what affects frequency and what affects voltage.
Now rephrase your questions in the light of the research you have to do to get up to speed.

BTW,
why, when you are budget limited, did you choose the super amp over the Low Tim?
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Old 3rd May 2008, 05:35 PM   #3
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So with th +/-40 V transformer I get about 28 volts DC which probably isn't enough to drive the amp. However, could I still drive the amp using 120 V ac rms from an ac plug or would this cause significant issues?

Retrospectively I should have done the low tim. The super amp was recommended by an audiophile friend of my father.
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Old 3rd May 2008, 06:47 PM   #4
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Are you saying the transformer is 40-0-40 or 20-0-20 ? The latter would give you ~+/-28VDC. The former in the mid 50s. Of course this may collapse if the transformer is undersized, but almost anything will get some sound.

From the nature of your questions this is a project that you are almost completely unprepared for. Did you go to class or at least open the textbook?
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Old 3rd May 2008, 09:22 PM   #5
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its 40-0-40 -- I'm gonna use this transformer temporarily to demonstrate the amplifiers behavior

yes I am in over my head - I had zero idea what I was getting into when I started and I have learned a lot about amplifier design but even more about the technical aspects of analog electronics. I should have gone with the low tim amp even better the ESP Proj 3 amp.

thanks for the help
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Old 3rd May 2008, 09:50 PM   #6
Nordic is offline Nordic  South Africa
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When rectifying the AC to DC you will end up with voltages equaling AC voltage times the square root of 2... (1.414)
so those 40VAC rails will end up being about 56V-0V-56V
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Old 4th May 2008, 08:47 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by ComradeConrad
its 40-0-40 --
=40Vac-0Vac-40Vac.
Start using the expected language or you'll fail your grading.
+-40V refers to DC voltage NOT AC voltage. You need to learn a lot more before you start learning about amplifiers.

Quote:
Did you go to class or at least open the textbook?
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Old 4th May 2008, 02:03 PM   #8
jaycee is offline jaycee  United Kingdom
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For junior design you should have built a small 30W amp or so. The Leach amps are advanced stuff. The P3A seems simple enough but has a hidden catch - the output stage is of a type that will easily oscillate if not layed out properly. I'd probably recommend Destroyer X's DX AMP for a beginner.

A transformer puts out AC. The secondaries are rated at AC voltage. If you have done basic power supply theory you will know about rectification and smoothing, and that AC voltage is given as an RMS voltage eg RMS voltage = peak voltage / 1.414 (the square root of 2).

Therefore when you use a bridge rectifier and smoothing capacitors, you end up with a DC voltage that is the peak AC voltage less about a volt drop caused by the rectifier's diodes. So, 40VAC would give about 40 * 1.414 = 56VDC when rectified. Under load this will probably sag to nearer 52VDC.

I'd recommend reading and digesting this:
http://sound.westhost.com/power-supplies.htm
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Old 4th May 2008, 02:26 PM   #9
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Jaycee - I'll have to disagree with you on the Leach amp as a forst project - with ready made boards it is a stuff and go project. My first amp project was a 4 channel Leach that went almost without a hitch. (I swapped rail leads on one channel, but those 100R "fuses" that Leach recommends saved my amp)

The superamp is a bit more difficult in that you have to make a board (Prof Leach gaive a layout to copy) and it has a higher parts count, but it still is a relatively easy project since all the design work is done for you. The har part is finding nearly obsolete parts.

I tend to look at things a bit differently than most. Some of the projects with boards available here are quite simple in that you just stuff in accordance with the silk screen and bolt it to your heat sink. The circuits may be complex, but that doesn't make putting a 1K resistor in the R1 spot any harder. If the output devices are not on the board then you have a bit of a potential complication, but using slow devices like Leach does minimizes the potential problems.

I find myself wondering if it is really ComradeTroll who started this thread, even though most trolls come with a know it all attitude.

Edit: Andrew, ComradeC just copied my laziness on tranformer designation.
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Old 4th May 2008, 05:33 PM   #10
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Quote:
Originally posted by ComradeConrad


However, could I still drive the amp using 120 V ac rms from an ac plug or would this cause significant issues?

I'm surprised no one picked up on this important tidbit.

Comrade,
Never attempt to operate an amp direct from AC mains - it's very dangerous.
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